Tachycardia is a condition in which the heart beats rapidly. Pathologic tachycardia is hemodynamically disturbing, causing the drop of systemic blood pressure. There are many types of pathologic tachycardias and the electrophysiology differentiates two major classes: supraventricular and ventricular tachycardias. Tachycardia is often the result of electrical feedback within the heart structures where the natural beat results in the feedback of an electrical stimulus which prematurely triggers another beat. There are several different cardiac pacing modes which may terminate tachycardia. The underlying principle in all of them is that if a pacemaker stimulates the heart at least once shortly after a heartbeat, before the next naturally occurring heartbeat at the rapid rate, the interposed stimulated heartbeat disrupts the stability of the feedback loop thus reverting the tachycardia to sinus rhythm. Such a pacemaker is disclosed in U.S. Pat. No. 3,942,534 which, following detection of tachycardia, generates a stimulus after a delay interval.
The most hazardous arrhythmia is ventricular tachycardia which may progress into life-threatening arrhythmia ventricular fibrillation. Because ventricular tachycardia is not always successfully treated and terminated by antitachycardia pacing, the implantable cardioverter-defibrillator is used to deliver a high energy pulse shock to cause cardioversion of ventricular tachycardia to sinus rhythm. Such an implantable device is disclosed in U.S. Pat. No. 4,614,192 having a bipolar electrode for R-wave sensing, the system utilizing heart rate averaging and a probability density function for fibrillation detection. A similar system for cardioversion is disclosed in U.S. Pat. No. 4,768,512 which has high frequency pulse delivery. All these systems deliver high energy shock through the special patch-electrodes such as described in U.S. Pat. No. 4,291,707. To simplify the surgical procedure, systems having a superior vena cava electrode and subcutaneous electrode, such as described in U.S. Pat. No. 4,662,377, have been developed. The supraventricular tachycardia caused by atrial flutter or fibrillation can be also treated by an implantable cardioverter such as described in U.S. Pat. No. 4,572,191.
The difficulty in electrotherapy treatment of tachycardia is that the implantable apparatus has to include means for accurately detecting pathologic tachycardia so as to deliver the electrotherapy pulses whenever the pathologic tachycardia occurs. A problem is that the heart rhythm increases its repetition rate physiologically whenever either physical or emotional stress occurs. The means for pathologic tachycardia detection must accurately differentiate the natural sinus tachycardia, which should not be treated by means of electrotherapy from the pathologic tachycardia which has to be treated. Therefore discrimination between normal and pathologic tachycardia on the basis of rate measurement is not reliable. To overcome this problem numerous methods of tachycardia detection have been developed which are applicable in implantable electrotherapy devices.
Such a system is disclosed in U.S. Pat. No. 4,475,551 wherein heart rate sensing as well as a probability density function are used to distinguish between ventricular fibrillation and high rate tachycardia. Another system is disclosed in U.S. Pat. No. 4,790,317 which can automatically recognize the pathologic rhythm by monitoring the pulse sequence representing the ventricular electrical activity. At least two sensing positions i.e. for each ventricular epicardial surface, are used, but more sensing points will obtain better discrimination between normal and pathologic rhythms.
The problems which may occur with such systems are susceptibility to electromagnetic interference and muscular noise, as well as improper gain of the heart beat detectors causing the undersensing of cardiac rhythm. Therefore some means for detecting of noise and means for automatic sensitivity adjustment are desirable. Therefore the implanted pacemaker noise rejection system described in U.S. Pat. No. 4,779,617, as well as the automatic sensitivity control systems disclosed in U.S. Pat. No. 4,766,902 and U.S. Pat. No. 4,768,511 have been developed.
The implantable cardioverting system usually includes a cardiac pacing system because of the occurrence of bradycardial events which follow the cardioversion high voltage pulse. There are also patients who suffer from pathologic tachycardia as well as from bradycardia, to be treated by cardiac pacing. Therefore a physiological sensor for control of the heart rate is desirable to obtain rate responsive pacing. It is also possible for the cardioversion device to have a dual chamber physiologic pacing function. In such a system, a sensor for atrial fibrillation detection would be important not only for the appropriate ventricular response to atrial rhythms, but also for differentiating supraventricular from ventricular tachycardia. There are many physiological control systems for rate responsive pacing, but only few of them can be used for tachycardia detection as well. As far as is known, none of these sensor systems can be used for ventricular tachycardia detection, rate responsive pacing, atrial fibrillation detection, pacing capture and for noise detection. The system disclosed in U.S. Pat. No. 4,774,950 has a circulatory systemic blood pressure measurement system which detects a drop of pressure in the case of pathologic heart rhythm. A similar system is described in U.S. Pat. No. 4,791,931 wherein the pressure is measured by means of arterial wall stretch detection. Another system disclosed in U.S. Pat. No. 4,770,177 adjusts the pacing rate relative to changes in venous blood vessel diameter that is measured by a piezoelectric sensor. The heart contractions change the ventricular chamber volume due to the inflow and outflow of blood thus varying the impedance within the chamber. Impedance measurement is used in the system described in U.S. Pat. No. 4,773,401 to obtain physiological control of pacing rate. Furthermore stroke volume and ventricular volume measurement are possible in the system described in U.S. Pat. No. 4,686,987 as well as in U.S. Pat. No. 4,535,774. All these systems indirectly measure the mechanical contraction of the heart which is a consequence of the electrical depolarization and which is influenced by the sympathetic and parasympathetic nervous system as well as by circulatory cathecholamines. The sympathetic stimulation and circulatory cathecholamines increase the speed of the contraction and therefore the hemodynamic forces are accordingly transferred to the circulatory system. In the case of pathologic rhythm having an electric depolarization disturbance, hemodynamics will be impeded.
The quality of the mechanical cardiac contraction significantly differs in normal and pathologic rhythms. Therefore a system for direct measurement of parameters of mechanical cardiac contraction would be desirable because it would obtain more exact physiological parameters, which may be used for a rate responsive pacing algorithm as well as for detection of different cardiac rhythms. The system disclosed in U.S. Pat. No. 4,784,151 has a conductive rubber tube whose changes in resistance are measured, which are caused by tube distension. In such a system the cardiac contraction energy is transformed into the hemodynamic energy and again into the mechanical distension movements.
The system disclosed in U.S. Pat. No. 4,763,646 discloses a pacing lead having a sensor for heart sound, pressure and acceleration. The sound and pressure are physically of the same origin and only the frequency spectra are different. Therefore it is easy to detect both parameters with the same transducer by filtering of its signal, where the signal at the output of the low-pass filter is pressure, while the signal at the output of the high-pass filter is sound.